1. Field of the Invention
The present invention relates to a focus state detection device in a camera or the like that uses an optimum TTL (through the lens) phase difference detection method and, in particular, it relates to a focus state detection device wherein the focussing state is detected at a plurality of regions in the shooting plane of the shooting lens (object lens).
2. Description of Related Art
The basic structure of a focus state detection device used in a camera or the like and using a TTL phase difference detection method is shown in FIG. 14. This focus state detection device is composed in order of a field mask 20, a condenser lens 30, a diaphragm mask 40, a re-imaging lens 50 and a photosensitive element 60 on the optical axis 0 of the shooting lens 1. The focus state detection optical system is comprised of the field mask 20, the condenser lens 30, the diaphragm mask 40, the re-imaging lens 50 and the photosensitive element 60. The exit pupil 10 of the focus state detection optical system is at a conjugal position with the diaphragm mask 40 via the condenser lens 30. In the case shown in the figure, the conjugal position coincides with the position of the shooting lens 1, the field mask 20 being positioned near the predicted focussing plane (film plane or the like).
Light rays that pass through the two regions 101 and 102 into which the exit pupil 10 is divided are composed into the primary image of the subject near the field mask 20 by the shooting lens 1. Light rays are extracted by the field mask 20, restricting the light rays from the subject that reach the condenser lens 30. Light rays that pass through the condenser lens 30 pass through the apertures 401 and 402 of the diaphragm mask 40, which similarly restrict unnecessary light rays, and are re-composed into secondary images on the element rows 601 and 602 in the photosensitive element 60 by the regular lens components 501 and 502 of the re-imaging lens 50.
In other words, behind the primary image composed by the shooting lens 1, two secondary images that are approximately the same as the primary image are re-composed onto the pair of rows of photosensitive elements 601 and 602 by the condenser lens 30 and a re-imaging optical system comprised of the diaphragm mask 40 and the re-imaging lens 50, the focus adjustment state of the shooting lens I being detected on the basis of the positional relationship between the two secondary images. The positional relationship between the two secondary images on the pair of rows of photosensitive elements 601 and 602 changes in accordance with the focus adjustment state of the shooting lens 1. For example, when the focus is located behind the predicted focussing plane of the shooting lens, the two secondary images become farther apart, whereas if the focus is in front of the plane, the two images become closer together. Accordingly, the proper focus state can be detected by comparing the outputs of the two rows of the photosensitive elements 601 and 602.
In a focus state detection device based on these basic principles, as explained above, in general the position of the exit pupil 10 is conjugal to the diaphragm mask 40 via the condenser lens 30. In other words, the two regions 101 and 102 of the exit pupil 10 through which light rays pass are inverted projection images of the two apertures 401 and 402 of the diaphragm mask 40 through the condenser lens 30. Hence, the regions have fixed positions and are separate and independent of each other.
Moreover, when a plurality of focus state detection regions are provided on the photographic field, this is also true of the position of the exit pupil corresponding to each of the focus state detection optical systems, all of them being symmetric relative to the optical axis 0 of the shooting lens 1.
However, with the described focus state detection device, the following problems arise.
Namely, the position of the exit pupil of the shooting lens actually mounted is not fixed, but varies depending on the type of shooting lens, there being many instances in which the position of the exit pupil of the shooting lens is not conjugal to the diaphragm mask 40 via the condenser lens 30.
Because the position of the exit pupil in actuality varies a great deal between shooting lenses, in the case of a focus state detection optical system having focus detection regions at positions off the optical axis 0 of the shooting lens along the primary image plane, i.e. in lateral areas away from the center of the photographic field, it becomes necessary to conduct focus state detection using light rays that pass through two regions of the exit pupil, which are extremely non-symmetric relative to the optical axis 0 of the shooting lens. Consequently, the problems arise that the symmetry of the two light rays is lost, a portion of the light rays being lost because of the efficacy of the apertures, thereby resulting in the creation of a so-called eclipse, making focus state detection impossible.
In particular, when the focus state detection regions are positioned in the lateral areas of the photographic field along a radial direction relative to the optical axis 0 of the shooting lens, the loss of symmetry in the two light rays is phenomenal, and the eclipse phenomenon arises easier than in focus state detection regions positioned along the circumferential direction relative to the optical axis 0 of the shooting lens.
This state is described in detail hereafter, with reference to FIGS. 1-8.
FIG. 1 is an oblique view showing the basic structure of a focus state detection optical system to which the present invention can be applied, whereas FIGS. 2-5 are frontal views showing details of the various components of the system, and FIGS. 6-8 are explanatory drawings showing the state of eclipse on the exit pupil of the shooting lens (object lens).
As shown in FIG. 1, the focus state detection optical system of this focus state detection device is provided in order with a field mask 21, a condenser lens 31, a diaphragm mask 41, a re-imaging lens 51 and a photosensitive element 61 positioned on the optical axis 0 of the shooting lens. In the state shown in the drawing, the field mask 21 and the diaphragm mask 41 are positioned off the optical axis 0 for clarity.
In the present example, a case is shown wherein focus state detection regions are situated at three locations, namely, in the lateral areas to the right and left and in the center of the photographic field. Accordingly, there are three focus state detection optical systems corresponding to the three focus state detection regions in these three locations. For example, the focus state detection optical system corresponding to the focus state detection region in the center is comprised of an aperture 21L of the field mask 21, a regular lens component 311 of the condenser lens 31, an aperture 41L of the diaphragm mask 41, a regular lens component 51L of the re-imaging lens 51 and a photosensitive element component 61L of the photosensitive element 61. Similarly, the focus state detection optical systems corresponding to the focus state detection regions separated from the center are comprised of similar components, as indicated by appended letters M and N.
Each single focus state detection optical system is generally comprised of two regions, one in the vertical direction and one in the horizontal (sideways) direction, and the parts of the field mask 21, diaphragm mask 41, re-imaging lens 51 and photosensitive element 61 of a single focus state detection optical system have a vertical component and a horizontal component. In this sense, the focus state detection device shown in FIG. 1 can be considered to have a total of six focus state detection optical systems.
FIG. 15 shows the relationship that the focus state detection regions have with the photographic field when viewed from the direction of the viewfinder of the camera (not shown in drawings). In this example, a focus state detection region VL, which extends in a vertical direction, and a focus state detection region HL, which extends in a horizontal direction, intersect to form a cross-shaped focus state detection region on the optical axis 0. In addition, a focus state detection region VM, which extends in a vertical direction, and a focus state detection region HM, which extends in a horizontal direction, intersect to form a cross-shaped focus state detection region to the left side of the screen, off of the optical axis 0. Furthermore, a focus state detection region VN, which extends vertically and a focus state detection region HN, which extends in a horizontal direction, intersect to form a cross-shaped focus state detection region to the right side of the screen, off of the optical axis 0.
The field mask 21 is placed near the predicted focussing plane of the shooting lens and restricts the regions where focus state detection is conducted in the photographic field. The field mask 21 has three apertures 21L, 21M and 21N as shown in the enlarged frontal view of the field mask 21 in FIG. 2. The aperture 21L is placed on the optical axis 0 of the shooting lens, while apertures 21M and 21N are placed at positions separated from the optical axis 0 of the shooting lens. In addition, apertures 21L, 21M and 21N are cross-shaped, being comprised respectively of approximately rectangular apertures 212, 214, and 216 in the vertical direction and apertures 211, 213 and 215 in the horizontal direction. A single rectangular aperture corresponds to one focus state detection region, so that, for example, apertures 212 and 211 correspond to two intersecting (substantially perpendicular) focus state detection regions. In addition, aperture 213 and aperture 215 correspond to focus state detection regions separated from the optical axis 0 of the shooting lens, which extend in a radial direction relative to the optical axis 0, while aperture 214 and aperture 216 correspond to focus state detecting regions separated from the optical axis 0, which extend in the circumferential direction relative to the optical axis 0.
As shown in FIG. 1, the condenser lens 31 is composed of three regular lens components 311, 312 and 313, in which the regular lens component 311 of the condenser lens 31 corresponds to the aperture 21L of the field mask 21, the regular lens component 312 corresponds to the aperture 21M and the regular lens component 313 corresponds to the aperture 21N, the condenser lens 31 collecting light rays from the shooting lens that have been restricted by the field mask 21.
As shown in the enlarged frontal view of the field mask in FIG. 3, the diaphragm mask 41 is composed of 12 apertures, namely apertures 41a, 41b, 41c and 41d (center component 41L) that restrict light rays corresponding to regular lens component 311 of the condenser lens 31, apertures 41e, 41f, 41g and 41h (side component 41M) that restrict light rays corresponding to regular lens component 312, and apertures 41i, 41j, 41k and 41l (side component 41N) that restrict light rays corresponding to regular lens component 313. Light rays from regular lens component 311 of the condenser lens 31 on the optical axis 0 of the shooting lens are divided in two in the two perpendicular directions of the apertures 41a and 41b and apertures 41c and 41d of the diaphragm mask 41. Similarly, light rays from the regular lens component 312 of the condenser lens 31 off the optical axis 0 of the shooting lens are divided in two in the two perpendicular directions of apertures 41e and 41f and apertures 41g and 41h of the diaphragm mask 41, light rays from regular lens component 313 of the condenser lens 31 off of the optical axis 0 of the shooting lens being divided in two in the two perpendicular directions of apertures 41i and 41j and apertures 41k and 41l of the diaphragm mask 41.
The re-imaging lens 51 is composed of regular lens components 51a, 51b, 51c and 51d (center component 51L) corresponding to apertures 41a, 41b, 41c and 41d of the diaphragm mask 41, regular lens components 51e, 51f, 51g and 51h (side component 51M) corresponding to apertures 41e, 41f, 41g and 41h of the diaphragm mask 41, and regular lens components 51i, 51j, 51k and 51l (side component 51N) corresponding to apertures 41i, 41j, 41k and 41l of the diaphragm mask 41, as shown in the enlarged frontal view of the re-imaging lens in FIG. 4.
The photosensitive element 61 is composed of element rows 61a, 61b, 61c and 61d (center component 61L), element rows 61e, 61f, 61g and 61h (side component 61M), and element rows 61i, 61j, 61k and 61l (side component 61N), as shown in the enlarged frontal view of the photosensitive element in FIG. 6. Light rays that pass through apertures 41L, 41M and 41N of the diaphragm mask 41 pass through regular lens components 51L, 51M and 51N of the re-imaging lens 51 and are guided to the corresponding element rows of the photosensitive element 61.
With a focus state detection device having the described structure, detection of the focussing of the shooting lens is conducted in the manner described below.
Light rays that pass through the shooting lens on the optical axis 0 of the shooting lens pass through the aperture 21L of the field mask 21 positioned near the predicted focussing plane of the shooting lens, through which the focus state detection region on the photographic field and the direction of the intensity distribution of the subject used in focus state detection are stipulated (ascertained). Light rays that pass through the approximately rectangular aperture 211 of the field mask 21 in the horizontal direction pass through the corresponding regular lens component 311 of the condenser lens 31 and are divided in two by apertures 41a and 41b of the diaphragm mask 41, the light rays divided in two by apertures 41a and 41b of the diaphragm mask 41 being re-composed into a pair of secondary images on element rows 61a and 61b of the photosensitive element 61 by the corresponding regular lens components 51a and 51b of the re-imaging lens. The focus position of the shooting lens is detected from the secondary images composed on element rows 61a and 61b of the photosensitive element 61 using the described principle.
Similarly, light rays that pass through the approximately rectangular aperture 212 of the field mask 21 in the vertical direction, which is perpendicular to the described approximately rectangular aperture 211, pass through the corresponding regular lens component 311 of the condenser lens 31 and are divided in two by apertures 51c and 51d of the diaphragm mask 41 and are re-composed into a pair of secondary images on element rows 61c and 61d of the photosensitive element by the corresponding regular lens components 41d and 41d of the re-imaging lens 41.
In this way, through the creation of a cross-shape by overlapping approximately rectangular apertures 211 and 212, focus state detection is possible from the intensity distribution of the subject in the two perpendicular directions, focus state detection being possible even when, for example, the contrast of the subject is in only one direction.
In addition, the focus position of the shooting lens can be similarly detected in the lateral areas of the photographic field by the focus state detection optical systems corresponding to apertures 21M and 21N of the field mask 21, which are provided off of the optical axis 0 of the shooting lens.
In FIG. 1, reference numbers 11, 12 and 13 designate exit pupils of the shooting lens, the image composed by superimposing exit pupils 11, 12 and 13 being comprised of the inverted projection image of an aperture of the diaphragm mask 41 via the condenser lens 31 at the same position.
Exit pupil 12 has a conjugal relationship with the diaphragm mask 41 via the condenser lens 31. Exit pupils 11 and 13 show how the position of the exit pupil in an actual shooting lens can vary depending upon the shooting lens, as described earlier. Exit pupil 11 is located at a position farther from the image plane of the shooting lens than is exit pupil 12, whereas exit pupil 13 is located at a position closer to the image plane of the shooting lens than exit pupil 12.
A frontal view of the inverted projection images of the apertures of the diaphragm mask 41 via the condenser lens 31 at the position of exit pupil 12 is shown in FIG. 6. As is clear from this figure and FIG. 3, aperture 41a of the diaphragm mask 41 corresponds to inverted projection image 12a, aperture 41b corresponds to inverted projection image 12b, aperture 41c corresponds to inverted projection image 12c, and aperture 41d corresponds to inverted projection image 12d, via regular lens component 311 of condenser lens 31.
In addition, by making the magnification of the regular lens components 311, 312 and 313 of the condenser lens 31 the same, the regular lens components 312 and 313 of the condenser lens 31 are positioned so that apertures 41e, 41f, 41g and 41h of the diaphragm mask 41 correspond to the inverted projection images 12a, 12b, 12c and 12d via regular lens component 312 of the condenser lens 31, and apertures 41i, 41j, 41k and 41l of the diaphragm mask 41 correspond to the inverted projection images 12a, 12b, 12c and 12d via regular lens component 313 of the condenser lens 31.
In other words, the inverted projection image 12a of exit pupil 12 is comprised of the inverted projection image of apertures 41a, 41e and 41i of the diaphragm mask 41 via the three regular lens components 311, 312 and 313 of the condenser lens 31.
Inverted projection images 12b, 12c and 12d are also each similarly comprised of the inverted projection images of three corresponding apertures of the diaphragm mask 41.
As shown in FIG. 7, the inverted projection images 13a, 13b, 13c, 13d, 13e, 13f, 13g, 13h, 13i, 13j, 13k and 131 of the apertures of the diaphragm mask 41 through the condenser lens 31 at the position of the exit pupil 13 correspond to apertures 41a, 41b, 41c, 41d, 41e, 41f, 41g, 41h, 41i, 41j, 41k and 41l of the diaphragm mask 41, respectively, the regular lens components 311, 312 and 313 of the condenser lens 31 being positioned so that these inverted projection images overlap at the position of exit pupil 12. Hence, at exit pupil 13, which is located at a position closer than exit pupil 12, inverted projection images 13e, 13f, 13g, 13h, 13i, 13j, 13k and 13l corresponding to the focus state detection regions positioned off of the optical axis 0 of the shooting lens are projected to a position off of the optical axis 0 of the shooting lens.
Consequently, out of the pair of inverted projection images 13e and 13f corresponding to the focus state detection regions positioned in the radial direction (see arrow R in FIG. 1) relative to the optical axis 0 of the shooting lens and provided off of the optical axis 0 of the shooting lens, inverted projection image 13f protrudes from the exit pupil 13, causing an eclipse to be created in the focus state detection light rays, thereby causing a substantial loss in the symmetry of the pairs of light rays, making focus state detection impossible.
As shown in FIG. 8, the inverted projection images 11a, 11b, 11c, 11d, 11e, 11f, 11g, 11h, 11i, 11j, 11k and 11l of the apertures of the diaphragm mask 41 through the condenser lens 31 at the position of the exit pupil 11 correspond to apertures 41a, 41b, 41c, 41d, 41e, 41f, 41g, 41h, 41i, 41j, 41k and 41l of the diaphragm mask 41, respectively, and similarly, out of the pair of inverted projection images 11e and 11f corresponding to focus state detection regions positioned in the radial direction (see arrow R in FIG. 1) relative to the optical axis 0 of the shooting lens and provided off of the optical axis 0 of the shooting lens, inverted projection image 11e protrudes from the exit pupil 11, causing an eclipse to be created in the focus state detection light rays, causing a substantial loss in the symmetry of the pairs of light rays, making focus state detection impossible.
In order to prevent the effects of eclipsing caused by the exit pupils of the shooting lens, which becomes a problem when focus state detection regions are provided in the lateral areas of the photographic field, Japanese Laid Open Patent Application No. 63-284513 discloses a well-known method whereby the surface area of the diaphragm apertures of the diaphragm mask is made smaller, and the two divided regions of the exit pupil are made smaller in a focus state detection optical system having focus state detection regions in the lateral areas of the photographic field.
Furthermore, Japanese Laid Open Patent Application No. 1-288810 proposes a method wherein the shape and the distance between the centers of the diaphragm apertures are varied between the lateral areas and the center of the focus state detection regions, the shape and the distance between the centers of the two divided regions of the exit pupil being varied in a focus state detection optical system having focus state detection regions in the center and in the lateral areas of the photographic field.
However, both of these methods were proposed in order to effect improvement relative to focus state detection optical systems having focus state detection regions positioned in the circumferential direction (refer to arrow P in FIG. 1) and off of the optical axis of the shooting lens, and are not methods for solving the lack of symmetry caused by eclipsing of the light rays passing through the two regions of the exit pupil in a focus state detection optical system, wherein the focus state detection regions are positioned in the radial direction relative to the optical axis of the shooting lens, as described above. Moreover, these methods give no indication relating to a plurality of focus state detection optical systems in which the various focus state detection regions are perpendicular and are positioned in the lateral areas of the photographic field.